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So, I downloaded and installed the campaign (put the .vpk in the bin folder to extract everything, put the folder resulting in the addons folder and all that fun crap) as it should, it showed up in the addons and I went to start it up. It went to the loading screen just fine but, just as it finishes loading, my mouse disappears, the loading screen disappears and it sends me back to my desktop. When I check the taskbar, L4D is closed. I've tried redownloading, reinstalling Steam and L4D but to no avail. I've also searched everywhere to see if anyone else is having this problem but it seems like I'm the only one.